This invention relates to an annulus fluorescent lamp that comprises a plurality of fluorescent tubes having different diameters and communicating with each other at a bridge portion.
It is known that a temperature of an electrode seal portion of a fluorescent lamp rises excessively when the lamp approaches the end of its life. A method for protecting against such an excessive temperature rise in the last period of the lamp life is disclosed in Japanese laid-open patent application (Tokukai-hei) 2-192650 or 4-19901, for example. In this method, a thermal fuse provided close to the end portion of the fluorescent tube fuses as the temperature of end portion rises excessively to cut off the drive circuit of the fluorescent tube.
Recently, a new type of annulus fluorescent lamp with high efficiency, compact size and light weight is proposed in Japanese laid-open patent application (Tokukai-hei) 2-61956, 6-203798 or 8-236074, for example. This type of annulus fluorescent lamp comprises two small fluorescent tubes having different diameters, disposed substantially concentrically in substantially the same plane. These fluorescent tubes are communicated with each other at a bridge portion so that a single discharge path is formed inside the fluorescent tubes. The lamp also comprises a high frequency inverter circuit to drive the fluorescent tubes.
Such an annulus fluorescent lamp has the following disadvantage to be improved. Generally speaking, in the last period of the life of a fluorescent lamp, an electron emitting substance of a filament is exhausted, so that a cathode voltage drop increases. As a result, a power loss at the filament (i.e., electrode) increases and the temperature at the electrode seal portion rises excessively.
In addition, some high frequency inverter circuits may continue to provide a preheat current to the fluorescent tube even if the fluorescent tube becomes unable to light due to the increasing cathode voltage drop. In this case, an arc discharge can occur between the inner lead wires connected to the electrodes, or an electrical breakdown can occur on the glass surface of the electrode seal portion that seals the inner lead wires. Such phenomena can occur if the current supply capability of the high frequency inverter circuit is high.
Especially, each of the above mentioned double annulus fluorescent tubes has a small diameter, and the electrode seal portion of the annulus fluorescent tube is covered with a plastic mouthpiece. Therefore, the temperature rise is remarkable and the cooling ability is not good at the electrode seal portion. Thus, the mouthpiece can be heated excessively to be distorted by the heat.
Furthermore, such a double annulus fluorescent lamp has a lowest temperature portion at a bridge portion side that is opposite to the electrode seal portion of the annulus fluorescent tube, and the heat of the electrode seal potion can be conducted to the lowest temperature portion easily since the electrode seal portion and the lowest temperature portion are covered with a single mouthpiece. If the temperature of the electrode seal portion rises in the last period of the life of fluorescent lamp, the lowest temperature portion is heated excessively. As a result, a mercury vapor pressure in the annulus fluorescent tube rises excessively out of a proper range, and the lamp luminosity and luminescence efficiency go down.
As explained above, the double annulus fluorescent lamp has a disadvantage, that is an excessive rise of temperature at the electrode seal portion in the last period of the life of a fluorescent lamp resulting in the distortion of the mouthpiece, and the decreases of the lamp luminosity and luminescence efficiency.